There are many surgical approaches and methods used to fuse the spine. Most involve the placement of a bone graft between the vertebrae. Supplemental hardware, such as plates, screws and cages may or may not be used, depending upon the indication.
An early cage design is described in U.S. Pat. No. 4,501,269 to Bagby, entitled “PROCESS FOR FUSING BONE JOINTS.” According to the method, a hole is bored transversely across the joint and a slightly larger cylindrical basket is driven into the hole, thereby spreading the bones in resistance to the tensile forces of the surrounding tissue. Immediate stabilization of the joint is achieved by the implantation of the rigid cylindrical basket. Subsequent bone-to-bone fusion is achieved, both through and about the basket, which is filled with bone fragments produced during the boring step.
The Bagby patent states that the process is applicable to any human or animal joint formed by opposed contiguous bony surfaces which are covered and separated by intervening cartilage and are surrounded by ligaments which resist expansion of the joint. Specific examples of such joints are a spinal joint between adjacent vertebrae or the ankle joint. This stand-alone interbody fusion technique continued to evolve with material changes and the design of threaded cages to increase stability and decrease displacement rates. Bilateral, parallel implants were designed for use in the lumbar spine, with the first human implantation occurring in the early 1009s. The cylindrical titanium cages were threaded to screw into the endplates, thereby stabilizing the device and allowing for increased fusion rate with a stand-alone anterior device.
Ray and colleagues developed a similar titanium interbody fusion device which was initially used in posterior lumbar interbody fusions (PLIF), but expanded to include ALIF procedures. In 1985, Otero-Vich reported using threaded bone dowels for anterior cervical arthrodesis, and femoral ring allograft bone has subsequently been fashioned into cylindrical threaded dowels for lumbar application.
Currently, there are a wide number of available interbody fusion devices of varying design and material, including:
1) Cylindrical threaded titanium interbody cages;
2) Cylindrical threaded cortical bone dowels; and
3) Vertical interbody rings, boxes and wedges.
All existing devices are prefabricated and not anatomically shaped, thus requiring the intervertebral space to be shaped to accommodate the device. This requires multiple steps and tools to prepare the area, distract, measure, and trial/size prior to ultimate placement. The requisite distraction devices and trial sizers interfere with the actual device for implant, creating more steps and decreasing the accuracy of placement. Other solutions, such as mesh cages and bone materials, are prone to deformation and/or breakage when attempts are made to force them into the discal interspace.
As a consequence of these limitations, only a small area of the endplate surface remains proximate to bone graft, which is dictated by the size and position of the cage. With current approaches, it is also difficult to visualize bone fusion mass because the cages and other devices are metal, and large trays are required with many instruments, due to the large number of different sized cages.